The 3rd Generation Partnership Project (3GPP) long term evolution (LTE) system, introduced as 3GPP release 8, is an improved universal mobile telecommunication system (UMTS). An LTE system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNodeBs) communicating with a plurality of mobile stations, referred as user equipment (UE). 3GPP introduces new features to help LTE system operators to further optimize network planning in a cost-effective way. Minimization of Drive Test (MDT) is one of the features where UEs collect measurements and report measurement information to their serving eNodeBs.
MDT has been worked on in 3GPP to help with network optimization. Network optimization is traditionally done by manual drive testing, which is costly and causes additional CO2 emissions. The essence of MDT feature is to provide for normal mobile terminals the possibility to record and log information relevant to the radio communication of the mobile terminals, correlated with the geographical location of the mobile terminals. MDT feature enables UEs to perform Operations, Administration, and Maintenance (OAM) activities, such as neighborhood detection, measurements, logging and recording for OAM purposes, which includes radio resource management (RRM) and optimization purposes.
Among the various MDT features, MDT can be used for uplink (UL) coverage analysis. According to the 3GPP release 11 (Rel-11 LTE) MDT work item, the UL coverage use case should be implemented according to the following objectives and requirements. The first objective is to identify weak UL coverage. The second objective is to perform coverage mapping of UL, i.e., to show measured UL radio performance and geographic location. The third objective is to identify whether UL coverage is limited by pathloss or interference conditions for overshoot, pilot pollution, and overlapping cells.
In Rel-10 LTE, power headroom (PH) measurement by UE (see 3GPP TS 36.213) was included for the MDT UL coverage use case. Power headroom report (PHR) is a mechanism typically applied for power control in OFDM communication systems. In OFDM systems, the transmit power of each UE needs to be maintained at a certain level and regulated by the network. The maximum transmit power of each UE, however, is different depending on UE capacity. PHR is thus used to configure the UE to provide its power capacity and usage to the network. A UE uses PHR mechanism to periodically provide its serving base station (eNodeB) with its PH value, which is defined as a power offset between a UE-configured maximum transmit power and a UE-calculated current UE transmit power. Based on the received PH value, the eNodeB can regulate the UE transmit power with proper resource allocation.
In addition to power control, PHR can be used for MDT UL coverage analysis. The current art, however, is insufficient for accurate UL coverage analysis. Consider that the PH measurements tagged by UE locations are the only information available to the MDT server. To identify weak UL coverage areas, the MDT server will examine those PHR with negative values carefully in order to find out whether the tagged locations are within weak UL coverage areas. However, as the available information is limited, it is difficult for the MDT server to make a good judgment on UL coverage. As a result, missed detections and false alarms may often occur.
For example, consider a UE that is in a weak coverage area and transmitting data with a small transport block size (TBS). The PH value reported by the UE may be non-negative due to the low transmit power required for small TBS. This leads to a missed detection. In another example, a false detection happens when a UE is in a good UL coverage area and sending data with large TBS or under a high interference level. The UE would need a high transmit power to maintain the communication quality, which may give rise to a negative PH value. A solution is sought to assist the MDT server to achieve the objectives and the requirements for determining UL coverage.